The invention relates to an optical switching element including two light wave guides, which are parallel and whose opposite walls are mutually separated by a small coupling distance, said light wave guides each being constituted by a strip of a semiconductor material having a first refractive index formed on a semiconductor substrate of a material having a second lower refractive index such that the light is confined in the strip constituting the guide, the dimensions of said guides being such that they each transport a monomode wave and the switching operation being initiated by the effect of the potential of the polarization means applied to the guides.
The invention moreover relates to an optical switching matrix constituted by a number nxn of these switching elements.
The invention is used in the switching of optical signals transported through optical fibres, for example in the telecommunication field, in which the switching between numerous optical fibres has to be rendered possible in order to avoid that the optical signals transported through the fibres have to be converted into electric signals during the switching between the different subscribers.
An optical switching element apt to form a switching matrix of the kind defined in the opening paragraph is known from the publication entitled "Low-loss GaAs-Ga.sub.1-x Al.sub.x As double-heterostructure directional coupler" by A. Carenco, P. Sansonetti et alii in "Technical Digest, Orlando Hyatt Hotel Kissimee, Florida, April 24-26, 1984" with the reference "Th B 4-1".
This document discloses a switching element constituted by two light wave guides constituted by strips of GaAs. These strips are parallel and are formed side by side on the surface of a layer of Ga.sub.1-x Al.sub.x As, which serves as a confinement layer between these light wave guides and the flat substrate of GaAs. Another layer of Ga.sub.1-x Al.sub.x As covers each of the strips constituting the guides. The latter are obtained on the other hand in relief on the substrate by etching of the starting layer. Electrodes of the Schottky type are finally formed on the surface of each of the guides, while the opposite surface of the substrate is provided with a layer forming an ohmic contact.
The guides each transport a monomode wave and due to the small distance laterally separating them a polarization applied to one or the other of the guides permits of changing the coupling of the two monomode guides, thus ensuring the desired switching operation.
A switching matrix can be obtained from such switching elements by joining the guides of one element to the guides of another element by means of curved guide portions.
However, such a structure has several disadvantages. In the first place, the coupling distance between the guides is to be understood to mean the lateral distance between the strips on the substrate. This distance is an extremely critical factor for the behaviour of the switching element. Now the lateral distance between two semiconductor tapes formed on a flat surface can be controlled only with difficulty and is at any rate always large. In the device described in the aforementioned document, this lateral coupling distance is of the order of 2 to 3 .mu.m. Since the coupling length, i.e. the longitudinal dimension of the light wave guides, is associated with the lateral coupling distance, this longitudinal dimension in this case is several millimeters, which is really extremely large with regard to dimensions of integrated devices.
Moreover, the curved guide portions interconnecting the switching elements so as to form a matrix necessarily establish this connection at a small angle, which results in that such a matrix occupies a very large surface area. On the other hand, the methods of etching or of epitaxial growth used to form these guide portions are always anisotropic, which results in that the external walls of these portions are rough or have steps leading to losses in these portions.
Now in the field concerned of telecommunication application, there is a demand for electrooptical circuits of increasingly higher performance, of increasingly more pronounced miniaturization and of increasingly lower cost, i.e. a simpler construction.